A computer disc drive includes one or more discs mounted on a hub for rotation about a spindle axis. The discs are typically coated with a magnetic medium for storage of digital information in a plurality of circular, concentric data tracks. A spindle motor rotates the hub and the attached discs to allow a head or “slider” carrying electromagnetic transducers to pass over each disc surface and read information from or write information to the data tracks.
The slider is typically formed from a ceramic block having a specially etched surface that forms an air “bearing” as the disc rotates beneath the slider. The lifting force provided by the air bearing surface causes the slider to lift off and “fly” a very small distance above the surface of the disc as the disc spins up to its operating speed. Although the fly height of the slider is only a fraction of a micron, this thin film of air between the slider and the disc prevents damage to the fragile magnetic coating on the surface of the disc.
The spindle motor commonly includes a fixed stator and a rotor that rotates with the hub and the attached discs about the spindle axis. The rotor/hub may alternatively rotate with a spindle shaft or the shaft may be stationary so that the rotor/hub rotates about the shaft. The stator continuously energizes the rotor to overcome wind resistance as well as friction in the spindle motor bearings as the rotor/hub spins at high speed.
A number of factors determine the speed at which data can be stored and read from the discs. These factors include the density of the data tracks on the discs as well as the speed of the spindle motor (i.e., the rotational speed of the discs). Indeed, it is highly desirable in the disc drive art to have high disc rotation speeds in order to reduce track access times. Typical spindle motor speeds include 7,200 revolutions per minute and beyond. However, increases in disc drive spindle motor speeds lead to increases in vibration levels of the entire disc drive which, in turn, increase head position errors caused by disc wobble or “non-repeatable run-out.” The increased vibration levels also lead to undesirably high acoustic levels emanating from the operating drive.
Disc drive spindle motors are subject to a variety of different vibration sources, including environmental sources as well as vibrations resulting from operation of the motor itself. Internal vibration sources include electromagnetic forces from the commutation pulses used to drive the rotor, as well as friction from the motor bearings and drag forces from the air passing over the rotating discs. Spindle motors, like all mechanical structures, are susceptible to certain natural resonant frequencies or “modes” where the vibration amplitude is increased due to the specific structural make-up of the motor. One of the most critical vibratory modes or resonant frequencies of a spindle motor is the “rocking” mode which leads to a rocking or wobbling displacement of the discs and the disc hub relative to the spindle axis. Indeed, rocking mode vibrations are believed to be a primary contributor to non-repeatable run-out.
Prior art disc drives offer minimal damping to attenuate rocking mode effects during operation of the drive. These undamped prior art drives suffer from relatively high amplitude vibrations at the rocking mode resonant frequencies. As noted above, large amplitude resonance vibrations lead to non-repetitive run-out which in turn causes track mis-registration. Of course, track mis-registration during a write operation can lead to data loss. Therefore, the ability of a disc drive spindle motor to reduce or absorb vibrations, and particularly rocking mode vibrations, has a significant impact on the performance of the drive (i.e., the ability of the drive to support high track and bit densities and fast spin rates) as well as the acoustic noise generated by the drive.
Accordingly, there is a need to reduce rocking mode vibrations in spindle motors to both increase the performance of the disc drive and reduce the acoustic level of the drive. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.